Tech Talk #82 – The 400-Horsepower Tune-up

By David Reher, Reher-Morrison Racing Engines

“A basic nitrous oxide injection system can make a huge difference in fast Sportsman racing.”

I’ve been around drag racing long enough that I can remember when racers first reached many of the sport’s performance milestones that are being celebrated during NHRA’s 60th anniversary season. There was a time when a 6-second elapsed time was sensational. Don Garlits was so elated to run in the sixes that he famously shaved his beard on the starting line after winning the 1967 U.S. Nationals. Today, however, six-second runs are commonplace – and they have become the price of admission in fast sportsman eliminators like Top Dragster and Top Sportsman.

Going fast is an essential part of drag racing for many participants. Certainly the growing popularity of Top Dragster, Top Sportsman, and Quick 32 competition at national and divisional events reflects this fact. Instead of the nitromethane and superchargers that were necessary to run sixes back in the day, most of today’s 6-second dragsters and doorslammers run the number with naturally aspirated engines.

It wasn’t too long ago that a 7-second e.t. was quick enough to make a 32-car field, but now it takes a 6-second time slip to qualify for one of the fast sportsman eliminators at many events. So how does a racer keep up with this escalation in performance without busting the racing budget? Installing a basic nitrous oxide system is a quick way to add 400 to 500 horsepower to a naturally aspirated engine.

Let’s say a racer has a 565 to 600-cubic-inch, naturally aspirated big-block that produces around 1200 horsepower – a typical combination in Top Dragster. Adding a simple two-stage nitrous system can easily boost the output to 1600 horsepower. To achieve a similar gain by increasing displacement or extending the engine’s operating range would be far more expensive.

The nitrous-injected engines we build for the NHRA Get Screened America Pro Mod Drag Racing Series are basically 854ci Pro Stock engines on steroids. They have elaborate four-state nitrous oxide systems that boost output by well over 1,000 horsepower. A Top Sportsman or Top Dragster engine doesn’t need anything as complex to run in the sixes.

While nitrous oxide injection does offer an instant performance boost, it’s a huge mistake to bolt a nitrous system onto an engine that’s not built for one. The basic assembly must be nitrous-compatible. If it’s not, the components won’t be able to withstand the higher loads and increased stress produced by nitrous-assisted combustion. If an engine is marginal on reliability at 1200 horsepower output, it will be a hand grenade at 1600 horsepower. When an engine burns up or fails catastrophically because its components aren’t up to the strain, then a nitrous system is no longer a bargain.

The U.S. government spent a great deal of time and money developing nitrous oxide injection. This research wasn’t for the benefit of drag racers – it was to improve the high-altitude performance of piston-engined fighter planes during World War II. The British and the Germans were working on similar programs because pilots’ lives and the outcome of the war hinged on building planes that could fly higher and faster than enemy aircraft. With the advent of jet engines, research on piston engines was no longer a priority in the military – but the knowledge that was learned transferred to other fields, including auto racing.

Nitrous oxide isn’t combustible and has no heat energy – but it is a fantastic oxidizer. The additional oxygen that nitrous oxide supplies to the combustion process allows an engine to burn more fuel – and that means it can convert more energy into mechanical force. An engine that burns 600 pounds of fuel per hour could burn 900 pounds of fuel per hour with the addition of nitrous oxide injection. This additional fuel volume represents a huge increase in potential performance.

Like a supercharger or a turbocharger, nitrous oxide increases the oxygen supply in the cylinder. While turbos and blowers add oxygen by mechanically pressurizing the induction system, nitrous adds oxygen chemically. When nitrous oxide breaks down under heat and pressure, it adds extra oxygen molecules to the mixture. Whether more oxygen is supplied mechanically or chemically, the result is an increase in combustion pressure that ultimately produces more power.

Just as you wouldn’t bolt a blower or a turbo onto an engine that wasn’t designed for forced induction, you shouldn’t install a nitrous system on an engine that’s not built to cope with the higher cylinder pressure it creates. A nitrous-injected motor needs bigger wrist pins, thicker pistons, and a different ring package than a conventional engine to withstand this higher load.

The nature of nitrous-assisted combustion also requires changes in the engine combination. In most naturally aspirated racing engines, the goal is to speed up combustion – but in a nitrous motor, the objective is to slow it down. Since nitrous is an excellent oxidizer, the cylinder pressure spikes quickly, which can overstress internal components. Consequently when we build an engine that’s intended for nitrous oxide injection, we lower the compression ratio, modify the combustion chambers, and spread the camshaft lobes. These steps can significantly reduce the possibility of preignition, detonation, and parts damage.

The power increase with nitrous is nearly constant across the engine’s rpm range. Unlike a carburetor that increases fuel volume as rpm and airflow increase, a nitrous system that produces a 400-horsepower increase at 5,000 rpm will produce the same 400-horsepower increase at 8,000 rpm. That’s because the volume of fuel that’s being added doesn’t change, unless another system is activated.

Tuning a nitrous-injected engine at the track therefore requires a somewhat different approach, since the timing of the injection stages can be adjusted to suit the track conditions. I generally recommend a two-stage system for Top Sportsman-type applications. This setup allows the car to launch with one system activated, then the second system is turned on farther down the track. Many delay boxes have the capacity to control multiple nitrous oxide systems, so in many instances, an auxiliary timer isn’t needed.

Racing a nitrous-injected engine also requires a change in routine at the track. The nitrous bottle should be weighed after every run to determine how much was used – it’s the change in weight, not the bottle pressure, that is an accurate gauge of how much nitrous was consumed. The nitrous oxide should be kept at a constant temperature with a bottle warmer to maintain consistent pressure. This is essential to maintain consistency when racing in a fast sportsman eliminator with a breakout.

Racing with nitrous oxide requires adjustments in how the engine is built and how a racer tunes at the track. With a nitrous-friendly engine and a thoughtful approach to setting up the system, a sportsman drag racer can go fast and have fun without hurting parts.